Session DC: Mini-Symposium on Experiment and Theory for Nuclei Close to the Driplines

Shapes of Exotic Nuclei

I will discuss how shapes may appear in exotic nuclei. Low-lying states of most of stable nuclei can be characterized by ``one shape.'' For instance, the ground state can be spherical with lowest states being phonon excitations, or the ground state forms an ellipsoid with lowest states being its rotation or gamma vibration. Such ``one shape for one nucleus'' picture leads us to the shape phase transition as functions of N and Z, with exceptions called shape coexistence. The situation of exotic nuclei may be different. The shell evolution changes their shell structure from that of stable nuclei so that nucleons can be excited from lower to higher orbits more easily, and states of different patterns of orbital configurations can coexist within a narrow energy region. Consequently, shape coexistence seems to occur in many exotic nuclei, not being too exceptional. I will show some examples by using recent results of Monte Carlo Shell Model. For exotic Ni isotopes, one sees a spherical ground state with superdeformed rotational band only 1-2 MeV lying higher. It is also of interest how the double magicity is preserved/broken in $^{56,68,78}$Ni. Thus, the shape of exotic nuclei may open a new window for theoretical and experimental studies. For the latter, new types of experiments clarifying band structures with wide variation may be relevant. Theoretical survey of such nuclei is of extreme and urgent interest, and in fact being carried out by combining simple shell-evolution scenario and large-scale calculations on supercomputers.   

The Nuclear Pairing Gap - How Low Can It Go?

The pairing gap for $^{53}$Ca obtained from new experimental data on the masses of $^{52-54}$Ca [F. Wienholtz et al., Nature {\bf 346}, 498 (2013)] has the smallest value yet observed. This is explained in the framework of the nuclear shell model with schematic and realistic Hamiltonians as being due to shell gaps around the low-$ j $ orbital $ 1p_{1/2} $ at $N=33$. I will also show comparisons of experiment and theory for the oxygen isotopes that have a small pairing gap at $N=15$ due to shell gaps around the low-$ j $ orbital $ 1s_{1/2} $. Minima in the pairing gaps for all nuclei are shown and discussed.   

Study of the Beta-delayed Neutrons from $^{77}$Cu using VANDLE

The Versatile Array of Neutron Detectors at Low Energy (VANDLE) measures the energy of neutrons emitted from nuclear excited states populated through beta decay or transfer reactions. The time-of-flight technique determines the energy, which requires a time resolution on the order of 1 ns. In addition, the detector requires a low threshold to measure neutrons at energies of 100 keV or lower. A successful experimental campaign at the Holifield Radioactive Beam Facility, using ions produced via proton induced fission of $^{238}$U, has yielded preliminary results on beta-delayed neutrons emitted during the decay of $^{77}$Cu. Of particular interest is the observation of low-energy neutrons emitted from states well above the neutron separation energy. Results from this experiment will be presented. This work was supported by the NNSA through DOE Cooperative Agreement DE-FG52-08NA28552.   

Beta-delayed neutron spectroscopy of the N=53 84Ga isotope with VANDLE

The advent of a new generation of radioactive ion beam facilities opens the study to experimentalists of neutron rich nuclei only found in stellar explosive nucleosynthesis. Interestingly, their decay properties are dominated by the large proton-neutron imbalances, giving rise to phenomena not seen close to stability. Theoretical calculations predict that in some cases the decay of deep core neutrons, the so-called Pigmy resonance, will dominate the decay strength. The Versatile Array of Neutron Detectors at Low Energy was developed at Oak Ridge National Laboratory/University of Tenessee as a high efficiency and low threshold neutron time-of-flight energy detector for $\beta$-decay studies. The beta-delayed neutron emission of thirty nuclei around doubly magic $^{78}$Ni and $^{132}$Sn was studied at the Holifield Radioactive Ion Beam Facility (ORNL). We will present preliminary results of the study of the $^{84}$Ga delayed neutron emission. A clear signature of the predicted Pigmy resonance was observed in the neutron spetrum at large excitation energy of 2 MeV. This work was supported by the NNSA through DOE Cooperative Agreement DE-FG52-08NA28552.   

Nuclear landscape and drip lines in covariant density functional theory

Neutron and proton drip lines represent the limits of nuclear landscape. While proton drip line is measured experimentally, the location of neutron drip line for absolute majority of elements is based on theoretical predictions which involve extreme extrapolations. The first ever systematic investigation of the location of proton and neutron drip lines in the relativitic Hartree-Bogoliubov (RHB) approach has been performed by us employing the set of modern covariant density functional parametrizations. Separable pairing is used in particle-particle channel of the RHB. This study covers all even-even nuclei with $Z\leq 120$ between proton and neutron drip lines. The accuracy of the description of the ground state (masses, two-particle separation energies, deformations, radii etc) properties of known nuclei and its dependence on parametrization have been analysed. Statistical errors in the predictions of neutron-drip line are established within the RHB. The comparison with the results of non-relativistic approaches (Skyrme density functional theory, macroscopic+microscopic approach) allows to define systematic errors in the predictions of neutron-drip line.   

Neutron-knockout reactions on beam of 106Cd

Studies of neutron knockout reactions on a beam of $^{106}$Cd will help understanding single-particle state evolution close to N$=$50 and add information to the level schemes of the reaction residues. Spectroscopic studies have been performed utilizing the S800 and CAESAR at the NSCL. These studies make use of single-, double-, and triple-neutron knockout reactions on beams of $^{106}$Cd. The momentum distributions of the resulting residues reflect the l-value of the removed neutron. Additionally gamma rays were measured in coincidence allowing for the separation of the knockout channel where the residue is left in an excited state from the channel to the ground state. Results from these studies will be presented and are used to validate the technique as used in the light tin isotopes.   

Neutron-knockout on beams of $^{106,108}$Sn

Characterizing the nature of single-particle states outside of double shell closures is essential to a fundamental understanding of nuclear structure. This is especially true for those doubly magic nuclei lying far from stability that are much less studied and where the shell closures influence nucleosynthetic pathways. The region around $^{100}$Sn is one of the most important due to the proximity of the N$=$Z$=$50 magic numbers, the proton drip-line, and the end of the rp-process. However, owing to low production rates, there is a lack of spectroscopic information and there are no firm J$^{\pi}$ assignments for odd-mass tin isotopes lighter than $^{109}$Sn. Single-neutron knockout experiments on beams of $^{106,108}$Sn have been performed at the NSCL. The combination of $\gamma$ ray measurements in CAESAR and momentum distributions from charged particles in the S800 allow the ground states of the beam particles and the final states of the residues to be characterized.